An RF cavity filter includes plural cavities formed therein to pass only use frequency band of a signal, and is employed generally at a base station, etc. using comparative high power of a frequency signal.
FIG. 1 is a plan view illustration structure of a common RF cavity filter.
In FIG. 1, the RF cavity filter includes an input connector 100, an output connector 102, a housing member 104, cavities 110, 112, 114, 116, 118 and 120 defined by the housing member 104 and a wall, resonators 130, 132, 134, 136, 138 and 140 in each of the cavities 110, 112, 114, 116, 118 and 120, and a coupling bar 160.
An RF signal inputted through the input connector 100 is provided to the cavity 110. Each of the cavities 110, 112, 114, 116, 118 and 120 and corresponding resonators 130, 132, 134, 136, 138 and 140 function as LC resonance elements, respectively.
The RF signal is delivered from one cavity to another cavity through a coupling window 150.
A resonance frequency of the filter is determined by size of the cavities and size of the resonators. A user may tune finely characteristics of the filter using tuning bolts which are not shown.
The skirt characteristic which means a slope of a boundary band in a pass band characteristic curve is important in view of the filter, and preferably should be formed sharply.
The skirt characteristic is improved according as order of the filter increases, i.e. the number of the cavities and the resonators increases. However, the skirt characteristic has trade-off relation with an insertion loss. That is, as the number of the cavities and the resonators increases, the skirt characteristic is enhanced but the insertion loss augments.
The filter forms a notch using cross coupling to improve the skirt characteristic with maintaining constant insertion loss.
The cross coupling means coupling between resonators which are not adjacent, e.g. coupling between a second resonator 132 and a fifth resonator 138. The cross coupling is realized generally through the coupling bar 160.
The coupling bar 160 is formed through a wall between the second cavity 122 and the fifth cavity 128, and is made up of a metal. The coupling bar 160 functions to deliver for example a signal of the second resonator 132 to the fifth resonator 138.
A hole for the coupling bar 160 is formed on the wall between the second cavity 122 and the fifth cavity 128, a dielectric layer is formed on an inner surface of the wall corresponding to the hole, and so the coupling bar 160 is not connected electrically to the wall.
However, this cross coupling structure may not adjust coupling amount between resonators. The coupling amount is determined by size of the coupling bar 160, and thus the coupling bar 160 should be replaced by new coupling bar having different size in case that desired coupling amount is not realized. In addition, the coupling amount may not be adjusted under the condition that the coupling bar 160 is set on the wall.
Moreover, the filter may not adjust transmission zero related to the skirt characteristic.